Process for the chemical-mechanical polishing of isolation layers produced using the STI technology, at elevated temperatures

ABSTRACT

A process for the chemical-mechanical polishing of isolation layers based on the shallow trench isolation (STI) technology, using a basic polishing slurry which contains from about 5 to about 12.5% by weight of a colloidal silica abrasive, characterized in that the polishing slurry is applied to the wafer surface at a temperature of about 35° C. to about 80° C., increases the polishing selectivity with regard to the rate at which silica is removed compared to the rate at which silicon nitride is removed.

BACKGROUND

[0001] The present invention relates to a process for the chemical-mechanical polishing of SiO₂ isolation layers produced using the STI (shallow trench isolation) technology, and in particular to a polishing process which is carried out at elevated temperatures.

[0002] Nowadays, chemical-mechanical polishing (CMP) is a preferred method in the fabrication of integrated circuits (ICs) in order to achieve global planarization on wafers. A wafer is a polished disc of silicon on which integrated circuits are constructed. First of all, a polishing slurry is applied to an elastomeric polishing pad or directly to the wafer surface which is to be polished. The polishing pad is then pressed against the surface which is to be polished and, in the process, is moved relative to the wafer plane, so that the particles of the polishing slurry are pressed onto the wafer surface. The movement of the polishing pad causes the polishing slurry to be distributed and therefore causes the particles on the wafer surface to be distributed, leading to chemical and mechanical removal of the substrate surface.

[0003] Polishing slurries can be divided into two categories. One category comprises a suspension of pyrogenic silica as abrasive, and the other category contains colloidal silica as abrasive. The methods for preparing the polishing slurries from pyrogenic silica and from colloidal silica, also known as silica sol, are different. The suspension of pyrogenic silica is obtained by dispersing pyrogenic silica in an aqueous medium. For polishing slurries which contain colloidal silica, the colloidal silica is produced directly, by means of the sol-gel technique, from an aqueous solution, e.g. from a sodium silicate solution. At no time during production is the colloidal silica in a dry state which may lead to agglomeration or aggregation, as is the case with the pyrogenic silica. The suspension of pyrogenic silica has a wider particle size distribution than the polishing slurry from the colloidal silica category. This leads to the particles of the polishing slurry comprising pyrogenic silica agglomerating or forming a sediment during storage and/or polishing, which additionally leads to a non-uniform particle size distribution. Therefore, when using the polishing slurry comprising pyrogenic silica, defects such as surface roughness and microscratches are produced on the polished semiconductor surface. The seriousness of this phenomenon increases if the line width of the IC component falls to 0.25 μm or 0.18 μm or below. Therefore, the polishing slurry belonging to the colloidal silica category is becoming increasingly widespread.

[0004] In integrated semiconductor technology, it is usually necessary for a plurality of active and passive elements within the integrated circuit structure to be isolated from one another. This is often achieved using the STI technique, which is able to resolve the problems of field oxide diffusion which occurs in the LOCOS process and of mechanical stresses and has the advantage of resulting in a good isolating action and of increasing the integration density and planarization of the IC component. Therefore, STI has become the principal isolation technique used for the 0.18 μm CMOS technology.

[0005] The STI technique comprises the generation of a narrow trench in the silicon, filling the narrow trench with silica (SiO₂), with the entire wafer surface at the same time being covered with a film of silica, followed by planarization using the CMP technique. It is customary for a harder silicon nitride (Si₃N₄) film previously to have been formed beneath the silica film which is to be polished, so that the silicon nitride film acts as a stop layer during the polishing. An ideal polishing slurry which is eminently suitable for use is, on the one hand, able to effectively polish the silica film above the narrow trench without, on the other hand, polishing off the silicon nitride film. This means that it is desirable to use a polishing slurry in which the rate of polishing of the silica film is as high as possible and the rate of polishing of the silicon nitride film is virtually zero.

[0006] An index which is customarily used to assess the rate of polishing of silica on silicon nitride is the polishing selectivity, which is defined by dividing the polishing rate of silica by the polishing rate of silicon nitride. If a polishing slurry with a low SiO₂/Si₃N₄ selectivity is used to polish the SiO₂ which is situated above the trench, what is known as dishing of the SiO₂ and erosion of the Si₃N₄ occur.

[0007] In current IC fabrication, the selectivity of the CMP polishing slurries used is not sufficiently high. Therefore, one shortens the duration of polishing and, to avoid the problem known as dishing of the SiO₂, uses the reactive ion etching (RIE) technique. However, this combined RIE+CMP process lengthens the overall production time by approximately 40% and therefore increases production costs.

[0008] Various polishing slurries have been developed for increasing the polishing selectivity with regard to the rate at which silica is removed compared to the rate at which silicon nitride is removed. U.S. Pat. No. 4,526,631 describes a polishing slurry comprising 6% by weight of colloidal silica, which is set to a pH at 22° C. of approximately 12 using KOH, with a polishing ratio of approximately 10 SiO₂ to 1 Si₃N₄. In U.S. Pat. No. 5,738,800, the polishing composition for polishing a combination of silica and silicon nitride contains an aromatic compound which forms complexes with silica and silicon nitride. In U.S. Pat. No. 5,759,917, the composition comprises ammonium cerium nitrate, acetic acid and pyrogenic silica. In U.S. Pat. No. 5,733,819, the polishing composition contains fine silicon nitride powder, water and an acid. In EP-A-853 335, the composition comprises pyrogenic silica as abrasive, a tetramethyl-ammonium salt and hydrogen peroxide. EP-A 853 110 provides an alkalized polishing slurry for improving the polishing selectivity, this slurry containing a fluoride salt.

[0009] Therefore, the object of the present invention is to solve the above problems and to provide a process for chemical-mechanical polishing which has a high polishing selectivity in terms of the rate at which the silica is removed compared to the rate at which silicon nitride is removed.

[0010] The above-mentioned object is achieved by a process for the chemical-mechanical polishing of isolation layers based on the STI technology, using a basic polishing slurry which contains from 5 to 12.5% by weight of a colloidal silica abrasive, characterized in that the polishing slurry is applied to the wafer surface at a temperature of 35° C. to 80° C.

[0011] After the STI technique has been carried out, i.e. after the narrow trenches have been produced and the silica has subsequently been deposited, the polishing in accordance with the present invention is carried out by applying a polishing slurry to the wafer surface at a temperature of from 35° C. to 80° C., in order to planarize the semiconductor surface. In particular, the process according to the invention is used to polish a composite material which contains silica and silicon nitride. In particular, the process is used to polish a dielectric film, such as for example a silica film, which is formed on a silicon nitride film. The silicon nitride film is used as a stop layer.

SUMMARY

[0012] The invention relates to a chemical-mechanical polishing process comprising applying a basic polishing slurry containing from about 5 to about 12.5% by weight of a colloidal silica abrasive to a wafer surface at a temperature ranging from about 35° C. to about 80° C. and polishing the wafer surface.

[0013] In one embodiment, the invention relates to a process comprising applying, at a temperature ranging from about 35° C. to about 80° C., a basic polishing slurry containing from about 5 to about 12.5% by weight of a colloidal silica abrasive to a wafer surface comprising a trench filled with SiO₂ and surface covered with a film of silica with and polishing the wafer surface. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.

DESCRIPTION

[0014] The novel feature of the present invention consists in the fact that the polishing is carried out at elevated temperature. The polishing temperature is preferably 65° C. to 75° C. The hydrolysis reactions of both silica and silicon nitride are exothermic. Therefore, when the polishing temperature is increased, the polishing rates of both silica and silicon nitride decrease. However, the polishing rate of silicon nitride decreases to a greater extent than that of silica. Therefore, the polishing selectivity with regard to the rate at which silica is removed compared to the rate at which silicon nitride is removed can be improved by increasing the polishing temperature.

[0015] With the polishing slurry used in the present invention, the colloidal silica abrasive is preferably present in a quantity of from 6 to 10% by weight. The colloidal silica may have a mean particle size of 10 nm to 1 μm, preferably 20 nm to 100 nm.

[0016] The mean particle size is determined in an ultracentrifuge.

[0017] The polishing slurry may furthermore contain 8 to 15% by weight, preferably 8 to 12% by weight, of a metal fluoride. Examples of particularly suitable metal fluorides are lithium fluoride, sodium fluoride and potassium fluoride.

[0018] The polishing slurry used in the present invention is basic. The pH at 22° C. of the polishing slurry is preferably higher than 10.5.

[0019] The following examples are intended to explain the process and the advantages of the present invention more completely, without restricting the scope thereof, since numerous modifications and variations will be evident to the person skilled in the art. All parts and percentages are by weight unless otherwise indicated.

EXAMPLES

[0020] The polishing slurries of the examples and comparative examples were produced in accordance with the instructions given below. The polishing slurries were used to polish films on silicon wafers by means of a Westech-372 polishing machine, the films comprising either low-pressure CVD silica (SiO₂) or low-pressure CVD silicon nitride (Si₃N₄). The results are given in Table 1. The polishing rate is calculated by dividing the difference in thickness before and after the polishing by the polishing duration, the film thickness being measured by Nanospec. The polishing selectivity is calculated by dividing the polishing rate of silica by the polishing rate of silicon nitride.

Example 1

[0021] Levasil® 50 CK, a colloidal polishing slurry of the silica category procured from Bayer AG, Leverkusen, was diluted with deionized water, resulting in a polishing slurry containing 7.5% by weight of colloidal silica. The mean particle size of the colloidal silica is 60 to 90 nm, and the specific surface area is 50 to 180 m²/g. The pH at 22° C. of the polishing slurry was 11.8. The polishing was carried out at 65° C. The results are given in Table 1.

Example 2

[0022] Levasil® 50 CK was diluted with deionized water, so that a polishing slurry containing 6% by weight of colloidal silica was obtained. 8% by weight of potassium fluoride was added to the dilute suspension and thorough mixing was carried out. The pH at 22° C. of the polishing slurry was 11.2. The polishing was carried out at 35° C. The results are given in Table 1.

Comparative Example 1

[0023] The same processes as in Example 1 were used, except that the polishing was carried out at 25° C. The results are given in Table 1.

Comparative Example 2

[0024] The same processes as in Example 2 were used, except that no potassium fluoride was added and the polishing was carried out at 25° C. The results are given in Table 1.

Comparative Example 3

[0025] SS 25, a polishing slurry procured from Cabot Microelectronics, Aurora, Ill., USA containing pyrogenic silica was diluted with deionized water, so that a polishing slurry containing 7.5% by weight of silica was obtained. The pH of the polishing slurry was 11.2. The polishing was carried out at 25° C. The results are given in Table 1.

Comparative Example 4

[0026] The same processes as in Comparative Example 3 were used, except that the polishing slurry employed contains 6% by weight of silica. The results are given in Table 1. TABLE 1 Silica Potassium fluoride Polishing concentration concentration temperature Polishing rate Polishing Examples Silica source (% by weight) (% by weight) (° C.) pH (Å/min) selectivity Example 1 Colloidal Silica 7.5% 0 65 11.8 1179  11 Example 2 Colloidal Silica 6%    8% 35 11.2 930 7.8 Comp. Ex. 1 Colloidal Silica 7.5% 0 25 11.8 755 3 Comp. Ex. 2 Colloidal Silica 6%   0 25 11.2 472 1.5 Comp. Ex. 3 Pyrogenic Silica 7.5% 0 25 11.2 780 2.5 Comp. Ex. 4 Pyrogenic Silica 6%   0 25 11.2 450 1.2

[0027] It can be seen from the above examples that, on account of the increase in the polishing temperature, the polishing selectivity of a polishing slurry belonging to the colloidal silica category can be increased in terms of the rate at which silica is removed compared to the rate at which silicon nitride is removed. Furthermore, the selectivity can also be improved through the addition of the metal fluoride.

[0028] The above description of the preferred embodiments of this invention has been given for reasons of explanation and description. Evident modifications or variations are possible in view of the above teaching. The embodiments have been selected and described in order to offer the best illustration of the principles of this invention and its practical application and, in this way, to enable the person skilled in the art to employ the invention in various embodiments and using various modifications which are appropriate to the specific use intended. All modifications and variations lie within the scope of the present invention. 

What is claimed is:
 1. A chemical-mechanical polishing process comprising applying a basic polishing slurry containing from about 5 to about 12.5% by weight of a colloidal silica abrasive to a wafer surface at a temperature ranging from about 35° C. to about 80° C. and polishing the wafer surface.
 2. The process according to claim 1, wherein the polishing slurry is applied at a temperature ranging from about 65° C. to about 75° C.
 3. The process according to claim 1, wherein the polishing slurry contains from about 6 to about 10% by weight of a colloidal silica abrasive.
 4. The process according to claim 1, wherein the polishing slurry contains from about 8 to about 15% by weight of a metal fluoride.
 5. The process according to claim 4, wherein the metal fluoride is selected from the group consisting of lithium fluoride, sodium fluoride and potassium fluoride.
 6. The process according to claim 5, wherein the metal fluoride is potassium fluoride.
 7. The process according to claim 1, wherein the process polishes a composite material which contains silica and silicon nitride.
 8. The process according to claim 1, wherein the pH at 22° C. of the polishing slurry is higher than about 10.5.
 9. The process according to claim 1, wherein the colloidal silica has a mean particle size ranging from about 10 nm to about 1 μm.
 10. A process comprising applying, at a temperature ranging from about 35° C. to about 80° C., a basic polishing slurry containing from about 5 to about 12.5% by weight of a colloidal silica abrasive to a surface of a wafer comprising a trench filled with SiO₂ and surface covered with a film of silica and polishing the wafer surface.
 11. The process according to claim 10, wherein the polishing slurry is applied at a temperature ranging from about 65° C. to about 75° C.
 12. The process according to claim 10, wherein the polishing slurry contains from about 6 to about 10% by weight of a colloidal silica abrasive.
 13. The process according to claim 10, wherein the polishing slurry contains from about 8 to about 15% by weight of a metal fluoride.
 14. The process according to claim 4, wherein the metal fluoride is selected from the group consisting of lithium fluoride, sodium fluoride and potassium fluoride.
 15. The process according to claim 5, wherein the metal fluoride is potassium fluoride.
 16. The process according to claim 10, wherein the process polishes a composite material which contains silica and silicon nitride.
 17. The process according to claim 10, wherein the pH at 22° C. of the polishing slurry is higher than about 10.5.
 18. The process according to claim 10, wherein the colloidal silica has a mean particle size ranging from about 10 nm to about 1 μm.
 19. In a shallow trench isolation process in which a trench is filled with SiO₂, a wafer surface is covered with a film of silica and in which planarization is carried out with a chemical-mechanical polishing, the improvement comprising applying to the wafer surface at a temperature ranging from about 35° C. to about 80° C. a basic polishing slurry containing from 5 to 12.5% by weight of a colloidal silica abrasive and polishing the wafer surface. 